Several medical conditions, for example, adult respiratory distress syndrome, laryngeal infections, interstitial lung disease, myocardial infarction, and general respiratory distress, require therapeutic administration of oxygen to compensate for deficient blood levels of oxygen. (See generally, Harrison's Principles of Internal Medicine, 13.sup.th ed., edited by Isselbacher et al., 1994). Such therapy can be as mild as constant release of oxygen via a tube placed just under or within the nostrils, or as drastic as mechanical forced ventilation for those who would otherwise be unable to breathe. In each case, practitioners have long been confronted with a well known side effect of this therapy: oxygen toxicity. Furthermore, in the case of mechanical ventilation, practitioners have also been concerned with barotrauma, or the physical disruption of cells and tissues due to unusually high air pressure within the lungs.
Practitioners of the art should suspect oxygen-induced lung injury in patients treated with high levels of oxygen (more than 50% oxygen, normal levels are about 21%) for extended periods of time, e.g., 5-7 days. Stogner et al., The Annals of Pharmacotherapy 26: 1554-1561, 1992. Animal and human studies indicate that the pathology of oxygen toxicity involves a latent period of 24-72 hours after hyperoxic exposure (100% oxygen), although evidence of injury has been seen in normal volunteers in as little as 3 hours. Sackner et al., Ann. Intern. Med. 82:40-43, 1975. The latent period is followed by an acute inflammatory phase marked by edema, alveolar hemorrhage, and inflammation, with variable degrees of necrosis of the pulmonary endothelium and type I pneumocytes. Stogner et al. The final phase involves hyperplasia of interstitial cells and type II pneumocytes, leading to hyaline membrane deposition, atelectasis, and pleural effusions. The end result of prolonged exposure to pure oxygen is death, which occurs in rats after 64 hours (on average) of exposure to 100% oxygen.
Oxygen toxicity is generally thought to be mediated by the action of reactive oxygen species ("ROS"). Such species include hydrogen peroxide, superoxide anion, singlet oxygen, and hydroxyl radical, which are produced in cell mitochondria as intermediates in the oxygen, and hydroxyl radical, which are produced in cell mitochondria as intermediates in the cytochrome oxidase system. (Stogner et al.). One to five percent of the oxygen entering this system is released as ROS. In some cells, especially inflammatory cells such as polymorphonuclear cells and macrophages, ROS are produced by additional cellular mechanisms, presumably as part of the body's anti-microbial defenses. Under normal oxygen conditions, cellular anti-oxidant defenses (such as described below) are sufficient to protect the cell from these reactive molecules. However, under hyperoxic conditions, the relative rise in ROS can overwhelm these defenses.
ROS are thought to cause damage by reacting with various molecules, including proteins, nucleic acids, and membrane lipids, thus destroying their functionality. Reactions with polyunsaturated fatty acids can lead to formation of lipid peroxides, which are powerful inhibitors of various enzymes and may undergo further reaction to form other reactive species. Certain ROS, especially hydroxyl radical, are particularly reactive, such that they nearly always react in the vicinity where they were formed. However, other species, such as hydrogen peroxide and super oxide anion, can diffuse away from the site of formation and cause further damage away from the site of formation.
A number of natural defenses against oxygen toxicity exist. These include catalase, which catalyzes the conversion of hydrogen peroxide to water and oxygen, superoxide dismutase (SOD), involved in the conversion of superoxide anion into hydrogen peroxide, and the glutathione redox cycle, which effects the reduction of both hydrogen peroxides and lipid peroxides. A number of other natural molecules, including vitamins A, C, and E, have anti-oxidant activity. Because one of the more important types of damage caused by ROS is peroxidation of polyunsaturated membrane lipids, lipid-soluble free radical scavengers such as vitamin E are thought to be particularly important in protecting against this kind of damage.
Inflammatory cells and cytokines play an important role in oxygen toxicity. For example, superoxide anion is a chemotactic agent for neutrophils and macrophages. These recruited cells can augment tissue destruction through release of inflammatory cytokines such as tumor necrosis factor (TNF) and interleukin-1 (IL-1) (which perpetuate the inflammatory response). Proteases such as elastase are also released, causing direct damage to tissues. Furthermore, neutrophils can release relatively large quantities of ROS through a mechanism called the respiratory burst, in which reactive oxygen species are generated within microsomes of the cell and released into the extracellular milieu.
The cytokine TNF plays a particularly significant role in oxygen toxicity. TNF has been shown to alter the mithochondrial respiratory chain so as to produce increased ROS. Baeuerle et al., Ann. Rev. Immunol. 12:141-179, 1994. This effect is toxic to cells lacking sufficient defenses against this rise in ROS, but the cytotoxic effect is suppressed in cells which overproduce manganese-dependent SOD (MnSOD). Interestingly, TNF itself induces the MnSOD gene. Thus, TNF potentially increases oxidative species while simultaneously increasing the cellular defense against such species.
Jensen et al. have shown that TNF directly mediates at least some of the toxicity of hyperoxia by showing that treatment with anti-TNF antibody improved the survival of mice exposed to high levels of oxygen. J. Appl. Physiol. 72(5):1902-1907, 1992. Furthermnore, pretreatment of the mice with sub-lethal doses of TNF prior to or soon after exposure to oxygen provided similar protection. This latter protection is thought to result from TNF's ability to induce MnSOD.
NF-.kappa.B is a factor closely associated with cellular responses to pathogenic conditions. Baeuerle et al. NF-.kappa.B up-regulates multiple inflammatory cytokines, such as TNF, and is activated by many of those same cytokines, including TNF. Baeuerle et al. Hydrogen peroxide was also found to activate NF-.kappa.B. As explained above, TNF induces MnSOD, which converts superoxide to hydrogen peroxide and would therefore provide a sufficient level of hydrogen peroxide to activate NF-.kappa.B, which in turn upregulates TNF. Thus, conceivably, hyperoxia could result in a self-perpetuating local inflammatory response.
The inflammatory component of oxygen toxicity is exacerbated by barotrauma caused by mechanical ventilation. Barotrauma has been described as cellular and ultrastructural pathologic changes in pulmonary parenchymal cells resulting from excessive intrapulmonary ventilatory gas pressure. Tsuno et al., J. Appl. Physiol. 69(3):956-961, 1990. Thus, mechanical ventilation has been shown to cause pneumothorax, pneumomediastinum, subcutaneous emphysema, and pulmonary cellular damage. Tsuno et al. Such trauma can induce an inflammatory response, including recruitment and activation of neutrophiIs. Jaeschke et al., J. lIeukocyte Biol. 61(6):647-653 (1997). Thus, mechanical ventilation can cause damage via several pathways.
Oxygen toxicity may well occur at normal atmospheric levels. Such a situation arises in individuals lacking the normal levels of anti-oxidants (SOD, glutathione, etc.) found in normal adults. This situation arises most commonly in premature newborns. For example, Jain et al. have shown that premature newborns have decreased glutathione levels. Pediatric Pulmonol. 20:160-166, 1995. Thus, premature newborns can be especially susceptible to respiratory distress as well as a condition called retinopathy of prematurity (ROP). ROP is characterized by retinal neovascularization eventually including the vitreous, possibly leading to retinal detachment and finally to blindness. A current hypothesis regarding the pathogenesis of this condition suggests that a hyperoxic atmosphere relative to the intrauterine environment disturbs the replication of mesenchymal spindle cells in the vanguard zone of the growing retina, causing formation of extensive gap junctions in these cells which may lead to release of angiogenic factors with subsequent neovascularization. Bossi et al., Intensive Care Med. 21:241-246, 1995. Of course, premature newborns suffering respiratory distress are likely to be administered oxygen, thus amplifying the problems caused by oxygen toxicity.
Currently there is no drug available that is effective in treating conditions resulting from hyperoxia. Attempts to treat these conditions have focused mainly on administration of oxygen radical scavengers such as SOD and Vitamin E, with mixed results. Stogner et al. Furthermore, conflicting conclusions were drawn from two different studies which tested the ability of a TNF inhibitor, pentoxifylline, to treat hyperoxia. Naureckas et al., Eur. Respir. J. 7:1397-1402, "Pentoxifylline does not protect against hyperoxic lung injury in rats," 1994. Lindsey et al., Journal of Surgical Research 56:543-548, "Pentoxifylline Attenuates Oxygen-Induced Lung Injury," 1994. Thus, there is a need in the art for a drug which will provide treatment for conditions resulting from hyperoxia.
An object of this invention is therefore to provide a therapeutic, prophylactic treatment for protecting against development of conditions resulting from hyperoxia, particularly, those conditions resulting from mechanical ventilation. A further object of this invention is to provide a treatment for oxygen toxicity and other conditions resulting from hyperoxia.